Circuit for simulating transformer winding hot spot temperature



May 13, 1958 T. c. LENNOX ETAL 2,834,920 CIRCUIT FOR SIMULATING TRANSFORMER WINDING HOT SPOT TEMPERATURE Filed June 29, 1953 [0W TEMPERHTI/RE (OffF/(lf/VT 0F 1755/5T/INC E RES/570R.

77/76. Inventors: hom s CLennoX, HOWSTClA.FO hThE|tZ, James Mckenney,

by fiwflz zzi Their/Attorney.

Unite States Patent nice 2,834,920 Patented May 13, 1958 CIRCUIT FOR SIMULATING TRANSFORMER WINDING HUT SPOT TEMPERATURE Thomas C. Lennox and Howard A. Fohrhaltz, Pittsfield, and James F. McKenney, Lenox, Mass, assignors to General Electric Qompany, a corporation of New York Application June 29, 1953, Serial No. 364,702

6 Claims. (Cl. 317-44) perature equal to the winding hot spot temperature rise over top oil temperature. The resulting temperature rise in the heater element is detected in any one of several ways, such as by the bulb of a liquid expansion or vapor pressure thermometer, a resistance temperature detector, or a thermocouple.

For usual applications, such as to power transformers where changes in the magnitude of the load current are not particularly fast or large, the relatively slow response of such a system is not a handicap. However, for certain applications, such as to grounding transformers, where fault currents of large magnitudes are suddenly applied, the relatively slow response of conventional systems makes it difficult or impossible to accurately simulate winding hot spot temperature for the first few minutes after the heavy load current is applied.

Accordingly, it is an object of this invention to provide for a grounding transformer an electrical circuit for substantially instantaneously simulating transformer winding hot spot temperature.

In a three-phase grounding transformer apparatus submerged within a dielectric liquid and having a neutral ground lead, our invention comprises a first resistance element having a high temperature coefficient of resistance submerged within said dielectric liquid, a current transformer inductively coupled to said neutral ground lead, a second resistance element having a low temperature coefiicient of resistance connected in series with said current transformer and said first resistance element, another transformer having two electrical windings, one winding of said another transformer connected across said second resistance element and the other winding of said another transformer having a relay coil and said first resistance element connected in series therewith.

The invention will be better understood by considering the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a view of a transformer having a conventional temperature detector connected thereto. Fig. 2 is an electrical circuit diagram of our invention. Fig. 3 is a temperature-time curve of an insulated copper conductor which is heated internally as by the passage of current therethrough. Fig. 4 is a temperature-time curve of a conventional temperature detector which is heated externally, as in a bath.

Referring now to the drawing and, more particularly, to Fig. 1, shown therein is a transformer apparatus com prising a magnetic core 10 and electrical windings 11 submerged within a dielectric liquid 12 within a tank 13.

A terminal bushing 14 is mounted on the cover of the tank and a secondary line lead 15 extends from windings 11 to bushing 14. To detect the hot spot temperature rise of the electrical windings 11 above the temperature of dielectric liquid 12 a current transformer 16 is inductively coupled to lead 15. Current transformer 16 sends a current proportional to load current through a heater element 17 submerged within dielectric liquid 12. The temperature rise of heater element 17 above the temperature of the dielectric liquid 12 is detected by suitable means, as by a resistance temperature detector 18 having an indicating dial.

As hereinbefore stated, such an arrangement for usual applications, as power transformers where changes in the magnitude of the load current are ordinarily not fast or large, is adequate. However, grounding transformers, where fault currents of large magnitudes are suddenly applied, the relatively slow response of a conventional temperature indicator system as in Fig. 1 makes it difficult or impossible to accurately simulate winding hot spot temperature for the first few minutes after a heavy load current is applied. This is graphically illustrated in Figs. 3 and 4. Fig. 3 is a curve of temperature rise versus time for a copper conductor such as heating coil 17 or the transformer winding 11. Fig. 4 is a curve of temperature rise versus time for a body such as temperature detector 18. By comparing Figs. 3 and 4, it will be seen that the temperature detector 18 lags in point of time the temperature of the windings 11 or heater element 17.

To overcome the slow response of the conventional system, we propose to measure the resistance of the heating coil 17 itself and convert said resistance into temperature values. In this way a substantially instantaneous response to winding hot spot temperature is obtained.

Referring now to Fig. 2, shown therein is a diagrammatic illustration of our invention. A resistor element 19 having a low temperature coeflicient of resistance is connected in series with a heater resistor element 20 having a high temperature coefiicient of resistance. The element 19 may be a material such as constantan and element 2!) may be a material such as copper. A transformer of appropriate ratio, as a 1:1 ratio transformer, has one of its windings 21 connected across element 19. The other winding 22 of the transformer has connected in series therewith a relay coil 23 and element 20. Connected across series connected elements 19 and 20 is a current transformer 24 which is inductively coupled to a neutral ground lead 25 of three-phase grounding transformer 26 having a conventional three-phase magnetic core (not shown). As in Fig. l, the heater resistance element 20 having a high temperature coefificient of resistance would be submerged in the dielectric liquid of transformer 26. The relay 23 may be adapted to operate a circuit breaker as illustrated in Fig. 2, or to serve some other desired function.

Normally no current flows in neutral ground lead 25. In the event of a ground fault condition on a phase of the circuit whose neutral is grounded through transformer 26, a large current flows through the windings of grounding transformer 26. The current transformer 24 causes a current proportional to the fault current to flow through series connected resistors 19 and 20. Current flow through resistors 19 and 20 beats them up. Voltage drops appear across resistors 19 and 20 and said voltages are impressed across windings 21 and 22. The windings 21 and 22 are connected to resistors 19 and 20 with such polarity that their voltage difference appears across relay 23. At first this voltage difference is insufficient to operate relay 23. If the fault condition persists, the windings of grounding transformer 26 heat up. Concomitantly the resistors 19 and 20 heat up. When resistors 19 and 20 heat up, the resistance of element 20 increases rapidly while the resistance of element 19 remains substantially constant. This leaves an effective voltage substantially equal to the difference between the voltages across elements 19 and 20 to energize the relay 23 and cause its contacts to open, or to perform some other desired function. Energization of relay 23 can be arranged to occur when the resistance of element 20 reaches a value in accordance with a predetermined temperature.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention, and that it is intended by the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Patent of the United States is:

1. In a three-phase grounding transformer apparatus having a magnetic core and electrical windings surrounding said core, said windings and core submerged in a dielectric fluid and a neutral ground lead extending from a neutral point in said windings to ground, an electrical circuit for simulating hot spot temperature in said windings, comprising two series connected resistance elements, another transformer having two electrical windings, one of said another transformer windings connected across one of said resistance elements and the other of said another transformer windings connected in series with the other of said resistance elements, and means for sending a current proportional to a fault current in said neutral ground lead through said series connected resistance elements, one of said resistance elements having a high temperature coefiicient of resistance and the other of said resistance elements having a low temperature co efficient of resistance, said high temperature coefficient Letters of resistance resietance element submerged in said dielectric fluid, the polarity of said other of said another transformer windings and the polarity of the voltage drop in the resistance element which is serially connected with said other winding being opposed.

2. In a three-phase grounding transformer apparatus comprising a magnetic core and electrical windings sur- 3. In a three-phase grounding transformer apparatus having a magnetic core and electrical windings surrounding said core, said windings and core submerged in a dielectric liquid, and a neutral ground lead extending from a neutral point in said windings to ground, an electrical circuit for simulating hot spot temperature in said windings, comprising two series connected resistance elements, one of said resistance elements having a high temperature coeflicient of resistance, and the other of said resistance elements having a low temperature coefficient of resistance, a current transformer inductively coupled to said neutral ground lead for causing a current proportional to a fault current in said three-phase grounding transformer windings to flow through said series connected resistance elements, said current transformer connected in series with said series connected resistance elements, said high temperature coeflicient of resistance resistance element submerged in said dielectric liquid, a 1:1 ratio transformer having two electrical windings, one of the windings of said 1:1 ratio transformer connected across said low tem perature coefficient of resistance resistance element, the other of said 1:1 ratio transformer windings having said high temperature coefficient of resistance resistance element and a relay coil in series therewith, said high temperature coefficeint of resistance resistance element and said 1:1 ratio transformer other winding having opposite polarities.

4. In a grounding transformer apparatus having a magnetic core, electrical windings surrounding said core and a ground lead extending from a neutral point in said windings to ground, electrical means for simulating hot spot temperatures comprising a first resistance means having a low temperature coefficient of resistance, a second resistance means having a high temperature coefficient of resistance, means for passing a current proportional to the current in said ground lead through said first and second resistance means, and electrical means detectings connected in series with the other of said resistance elements so that the voltages appearing across said other resistance element and said other winding when a current is flowing through said series connected resistance elements will oppose each other, one of said resistance elements having a high temperature coefficient of resistance, and the other of said resistance elements having a low temperature coefficient of resistance, said high temperature coefficient of resistance resistance element submerged in said dielectric liquid, a relay coil connected to be responsive to the voltage difference of said series con: nected other resistance element and other winding, and a current transformer inductively coupled to said neutral ground lead and connected across said series connected resistance elements whereby a current proportional to a fault current in said three-phase transformer will flow through said series connected resistance elements.

ing the difference in voltage drop appearing across said first and second resistances.

5. In a grounding transformer apparatus having a magnetic core, electrical windings surrounded said core, and a ground lead extending from a neutral point in said windings to ground, electrical circuit means for simulating hot spot temperatures comprising a first resistance means having a low temperature coefficient of resistance connected in series with the second resistance means having a high tem perature coefficient of resistance, means for passing a current proportional to the current in said ground lead through said first and second resistance means, and electrical means detecting the difference in voltage drop appearing across said first and second resistances.

6. In a grounding transformer apparatus having a magnetic core, electrical windings surrounding said core, and a ground lead extending from a neutral point in said windings to ground, electrical circuit means for simulating hot spot temperatures comprising a first resistance means having a low temperature coefficient of resistance, a second resistance means having a high temperature coefficient of resistance, means for passing a current proportional to current in said ground lead through said first and second resistance means, and electrical means in series with one of said resistances providing a voltage proportional to but having polarity opposite to the voltage across the other said resistance.

Jackson July 2, 1946 Camilli et a1 Dec. 20, 1949 

